Depends on preference mostly and what type of shooting. I was trained both eyes open for things like raiding houses and cqb for situational awareness but when I shoot for precision with a scopes rifle I will close my left eye

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When I first started shooting pistol last year I used the one eye method. It worked but as mentioned above my eye got tired quickly. Started practicing at the house with both eyes open. Had somewhat of a double vision sine I had the pistol on one side. With practice I quickly learned which object was my intended target.

Another thing to learn is keeping the front site in focus.

When I got the scope for the rifle I have been keeping both eyes open and it has been working out pretty good. Every once in a while I notice I was closing one eye with the scope though. Could be a habit thing.